1. Field of the Invention
The present invention related to a display device including a pixel array portion in which pixel circuits (referred to as “pixels” as well) each having an electro-optic element (referred to as either “a display element” or “a light emitting element” as well) are disposed in a matrix. More particularly, the invention relates to an active matrix type display device in which pixel circuits each having an electro-optic element as a display element having a luminance adapted to change depending on a magnitude of a drive signal are disposed in a matrix, and which includes an active element every pixel circuit, display drive being carried out in units of pixels by the active elements.
2. Description of the Related Art
A display device using electro-optic elements as display elements of pixels is known. In this case, a luminance of the electro-optic element is adapted to change depending on a voltage applied thereto or a current caused to flow therethrough. For example, the electro-optic element having a luminance adapted to change depending on the applied voltage is typified by a liquid crystal display element. On the other hand, the electro-optic element having a luminance adapted to change depending on the flowing current is typified by an Organic Electro Luminescence (an organic EL or an Organic Light Emitting Diode (OLED) which will be referred hereinafter to as “an organic EL”) element. An organic EL display device using the latter organic EL element is a so-called self emission type display device using the electro-organic element, as a self emission element, as the display element of the pixel.
The organic EL element is an electro-optic element utilizing a phenomenon that when an electric field is applied to an organic thin film, the organic thin film emits a light. The organic EL element has the less power consumption because it can be driven by a relatively low applied voltage (for example, 10 V or less). In addition, the organic EL element is a self emission element which self-emits a light, which results in that weight-lightening and thinning are readily carried out because a subsidiary illumination member such as a backlight necessary for the liquid crystal display device is not required for the organic EL display device. Moreover, no residual image occurs in a phase of display of a moving image because a response speed of the organic EL element is very high (for example, about several micron-seconds). In recent years, planar self emission type display devices each using the organic EL element as the electro-optic element have been actively developed.
Now, in the display devices each using the electro-optic element, including the liquid crystal display device using the liquid crystal display element, and the organic EL display device using the organic EL element, a passive matrix system and an active matrix system can be adopted as the system for driving the same. However, although the display device utilizing the passive matrix system has a simple structure, it involves a problem that it is difficult to realize a large and high definition display device, and so forth.
For this reason, in recent years, the active matrix system has been actively developed. In this case, in the active matrix system, a pixel signal which is supplied to a light-emitting element provided inside a pixel is controlled by using an active element similarly provided inside the pixel, for example, an insulated gate field-effect transistor (in general, a Thin Film Transistor (TFT)) as a switching transistor.
Here, when the electro-optic element within provided a pixel circuit is caused to emit a light, an input image signal which is supplied through a video signal line is fetched in a storage capacitor (referred to as “a pixel capacitor” as well) provided in a gate terminal (control input terminal) of a drive transistor by a switching transistor (referred to as “a sampling transistor”). Also, a drive signal corresponding to the input image signal thus fetched in is supplied to the electro-optic element.
In the liquid crystal device using the liquid crystal element as the electro-optic element, since the liquid crystal display element is an element of a voltage drive type, the liquid crystal display element is driven by using a voltage signal itself corresponding to an input image signal fetched in the storage capacitor. On the other hand, in the organic EL display device using an element such as the organic EL element as the electro-optic element, a drive signal (voltage signal) corresponding to the input image signal fetched in the storage capacitor is converted into a current signal (drive current) by using a drive transistor, and the resulting drive current is supplied to the organic EL element or the like.
In the electro-optic element of the current drive type typified by the organic EL element, when a drive current value differs, an emission luminance differs accordingly. Therefore, in order to cause the electro-optic element to emit a light with a stable luminance, it is important to supply a stable drive current to the electro-optic element. For example, the drive system for supplying the drive current to the organic EL element can be roughly classified into a constant current drive system and a constant voltage drive system. Since both the constant current drive system and the constant voltage drive system are the well known techniques, there is given none of the known literary documents describing the constant current drive system and the constant voltage drive system.
The organic EL element has voltage vs. current characteristics having a large gradient. Thus, when the constant voltage drive is carried out, a slight dispersion of the voltages or a dispersion of the element characteristics causes a large dispersion of current, thereby causing a large dispersion of luminance. Therefore, in general, there is used the constant current drive in which a drive transistor is used in a saturated region. Of course, with the constant drive as well, a current fluctuation causes a luminance dispersion. However, a small current dispersion only causes a small luminance dispersion.
Conversely, even with the constant current drive system, in order to hold the emission luminance of the electro-optic element constant, it is important that a drive signal written to and held in a storage capacitor is constant in correspondence to an input image signal. For example, in order to hold the emission luminance of the organic EL element constant, it is important that the drive current corresponding to the input image signal is constant.
However, a threshold voltage of an active element (drive transistor) for driving the electro-optic element, and a mobility of a carrier therein disperse due to the process fluctuation. In addition, the characteristics of the electro-optic element such as the organic EL element fluctuate with time. In general, when a low-temperature polysilicon TFT substrate or the like is used, the threshold characteristics and mobility characteristics of the transistor largely disperse. Even with the constant current drive system, such a dispersion of the characteristics of the driving active element, and such a fluctuation of the characteristics of the electro-optic element exert an influence on the emission luminance.
In order to cope with such a situation, for the purpose of uniformly controlling the emission luminance over the entire picture of the display device, various mechanisms for correcting the luminance fluctuation due to the fluctuation of the characteristics of the driving active element and electro-optic element described above within each of pixel circuits are investigated. One of these mechanisms, for example, is described in Japanese Patent Laid-Open No. 2006-215213 (hereinafter referred to as Patent Document 1).
For example, in the mechanism described in Patent Document 1, a threshold correcting function, a mobility correcting function, and a bootstrap function are proposed for a pixel circuit for an organic EL element. In this case, the threshold correcting function is provided for holding a drive current constant even when there are the dispersion and the temporal change in threshold voltage of a drive transistor. The mobility correcting function is provided for holding the drive current constant even when there are the dispersion and the temporal change in mobility of the drive transistor. Also, the bootstrap function is provided for holding the drive current constant even when there is the temporal change in current vs. voltage characteristics of the organic EL element.
In order to realize the threshold correcting function, the mobility correcting function, and the bootstrap function, a sampling transistor or each of transistors added for the threshold correction and the mobility correction needs to be turned ON or OFF at a predetermined timing by using a pulse signal.
It is noted that at the realization of the threshold correcting operation and the mobility correcting operation, the various mechanisms are devised for a configuration of a pixel circuit or a drive timing. Sometime a time period of threshold correction, and a time period for mobility correction are determined based on only an ON time period or an OFF time period of one transistor, otherwise they are determined based on an overlap time period of ON time periods, OFF time periods or an ON time period and an OFF time period of two transistors.
In addition, with regard to mechanisms described in Japanese Patent Laid-Open Nos. 2005-197202, Hei 05-299177, 2006-113376, 2005-158583, and 2003-316291, respectively, various techniques about a pixel layout are proposed.